1. Field of the Invention
This invention relates to a novel secondary battery and particularly to a small, light secondary battery.
2. Description of the Prior Art
In recent years, electronic devices have been remarkably reduced in size and weight and, as a natural consequence, a need for batteries, i.e. power sources, to come in proportionately reduced sizes and weights. In the field of primary batteries, reductions in size and weight have already made, e.g., as in lithium cells. Since these are primary batteries, they cannot be recharged for repeated use and, therefore, have found utility in limited applications. In the field of secondary batteries, lead acid batteries and nickel-cadmium batteries have heretofore been used. These two types of secondary batteries both have posed a serious problem with respect to reduction in size and weight. From this point of view, nonaqueous secondary batteries have been arousing great interest but still need to be fully developed for practicability. One of the reasons for the lack of practicability is that none of the active materials so far developed for use in the aforementioned secondary batteries satisfy such practical properties as cyclicity and self-discharging.
Meanwhile, a new group of electrode active materials which make use of the phenomenon of intercalation or doping of a layer compound, a form of reaction substantially different from the reaction occurring in the conventional nickel-cadmium batteries and lead acid batteries, have come to attract growing attention.
Since these new electrode active materials involve no complicated electrochemical reaction during the course of recharging and discharging, they are expected to have a highly advantageous cyclicity of recharging and discharging.
As examples of the electrode active material making use of the intercalation of a layer compound, chalcogenide type compounds which possess a lamelar structure are attracting attention. For example, such chalcogenide compounds as Li.sub.x TiS.sub.2 and Li.sub.x MoS.sub.3 exhibit relatively satisfactory cyclicity but possess such low magnitudes of electromotive force that their practical discharge voltage is about 2 V at most even when Li metal is used as a negative electrode. With respect to the high electromotive force which constitutes one of the characteristics of nonaqueous batteries, therefore, these compounds are not satisfactory. Such metal oxide type compounds as Li.sub.x V.sub.2 O.sub.5, Li.sub.x V.sub.6 O.sub.13, Li.sub.x CoO.sub.2, and Li.sub.x NiO.sub.2 are attracting attention in respect that they are characterized by possessing high magnitudes of electromotive force. These metal oxide type compounds, however, are deficient in cyclicity and utility, namely the proportion in which the compounds are utilized for actual recharging and discharging, and further in terms of the factor of overvoltage involved during the course of recharging and discharging. They have not yet been fully developed to the level of practicability.
Particularly, such secondary-battery positive electrodes of Li.sub.x CoO.sub.2 and Li.sub.x NiO.sub.2 as disclosed in Japanese Patent Application Laid-open No. 136131/1980 possess magnitudes of electromotive force exceeding 4 V when Li metal is used as a negative electrode and exhibit surprisingly high magnitudes of theoretical energy density (per positive electrode active material) exceeding 1,100 WHr/kg. They nevertheless possess low proportions available for recharging and discharging and provide levels of energy density falling far short of theoretical values.
As one example of the electrode active material utilizing the phenomenon of doping, a secondary battery of a new type using an electroconductive macromolecular compound as an electrode material is disclosed in Japanese Patent Application Laid-open No. 136469/1981. The secondary battery using this electroconductive macromolecuar compound, however, entails serious outstanding problems such as instable properties evinced by low cyclicity and large self-discharge and has not yet reached the level of practicability.
In the specifications of Japanese Patent Application Laid-open No. 35881/1983, No. 173979/1984, and No. 207568/1984, it is proposed to use large surface-area carbon materials, like activated carbon, as electrode materials. These electrode materials have been found to manifest a specific phenomenon which, unlike the phenomenon of doping, is presumably ascribable to the formation of an electric double layer due to their large surface areas. They are claimed to manifest conspicuous properties particularly when they are used in positive electrodes. It is further stated that they are usable partly in negative electrodes. When these large surface-area carbon materials are used in negative electrodes, however, they have serious drawbacks in cyclicity and self-discharging. Moreover, the utilization coefficient, i.e. the proportion of electrons (or paired cations) reversibly released or received per carbon atom, is extremely low, even falling below 0.05 and generally falling in the range of 0.01 to 0.02. This fact implies that when these materials are used in negative electrodes of secondary batteries, the negative electrodes become very large both in weight and volume. This point poses a serious obstacle on the way to actual adoption of the materials.
The specification of Japanese Patent Application Laid-open No. 209864/1983 discloses as electrode materials such carbonaceous materials as carbonated phenolic fibers whose hydrogen atom/carbon atom ratio falls in the range of 0.33 to 0.15. It discloses, that the carbonaceous materials manifest particularly desirable properties when they are used as positive electrode materials p-doped mainly with anions and that they are also usable as negative electrode materials n-doped with cations. These materials have a serious disadvantage that they are deficient in cyclicity and self-discharging when they are used as n-doped negative electrodes. They have another serious disadvantage in that they possess extremely low utilization coefficients and lack practicability.
It has long been known to use graphite layer compounds as electrode materials for secondary batteries. It has been known the art that graphite layer compounds having incorporated therein such anions as Br.sup..crclbar., ClO.sub.4.sup..crclbar., and BF.sub.4.sup..crclbar. ions are usable as positive electrodes. It is naturally conceivable that graphite layer compounds having incorporated therein such cations as Li.sup..sym. ions are usable as negative electrodes. In fact, the specification of Japanese Patent Application Laid-open No. 143280/1984 discloses adoption as negative electrodes of graphite layer compounds which have cations incorporated therein.
The graphite layer compounds which have incorporated therein cations, however, are highly unstable. Particularly, the fact that they possess extremely high reactivity with electrolytes is evident from the report published by A. N. Dey, et al. in the "Journal of Electrochemical Society," Vol. 117, No. 2, pp 222-224, 1970. When the graphite which is capable of forming a layer compound, is used as a negative electrode, this negative electrode is hardly practicable because it lacks stability of the self-discharging property necessary for a battery and shows an extremely low utilization coefficient.
The new group of electrode active materials which utilize the phenomenon of intercalation or doping in their current conditions are such that the properties which are inherently expected of the materials have not yet been realized from the practical point of view.